Many elements, such as nitrogen, phosphorous, arsenic, antimony, tin, bismuth, fluorine, chlorine, bromine, iodine, and boron can act as flame-retardants and, if when incorporated in selected chemical structures they become activated during an overheat condition, they can effectively interfere with the combustion reactions.
Flame-retardant additives vary widely in thermal stability, their effects on the flow behavior of the organo-polymer, and in the corrosivity and toxicity of the products of thermal and hydrolytic decompositions. Thermal stability not only affects the eventual capacity of the additive as a fire-retardant but also the more immediate consequences of corrosivity of the decomposition is premature. The liberation of corrosive gases is characteristic of the halogenated additives, flow behavior by the type of additive, i.e., non-plasticizing vs plasticizing, and toxicity to the action of specific chemical by-products on living tissue; in particular, toxicity is complicated by relationships to the conditions of pyrolysis such as temperature, length of heating time, oxidizing or reducing, as well as material composition.
The elimination of halogenated compounds was considered desirable as they are major contributors to corrosivity as well as toxicity; their by-products are represented by phosgene, halogen acids, and related compounds. The elimination of antimony oxide, particularly in conjunction with a halogen, was considered desirable to reduce both the toxicity and corrosivity of the antimony trichloride and/or the oxychloride formed at temperatures above approximately 600.degree. F. Phosphate esters contribute to reduced thermal stability, high flow indices and the concomitant possibility of fire spread, with secondary effects of toxicity and corrosivity of the vapor phase products.
In assessing the characteristics of fire-retardant additives, the degree of development of smoke, or smoke obscuration, must be considered as this can reduce vision and lead to panic in a fire situation. In this regard, smoke is considered to be an airborne mixture of heated gases, liquid droplets, and solid particles; in addition to reducing vision, it may act as an irritant causing tissue injury by heat and chemical reaction. Thus, by illustration, although adequate retardency of flame propagation can be achieved using mixtures of antimony oxide, a brominated epoxy and a halogenated plasticizer, the resultant smoking can be so heavy as to preclude its acceptability for use in aircraft interiors.
A melamine pyrophosphate, used as an aqueous suspension, is reported in U.S. Pat. No. 2,464,342 (1949) for imparting flame resistance to textile fibers. The degree of improvement obtained by the subject disclosure when benzenephosphonic acid was substituted as the acidic hydrogen donor for the melamine amine group was unexpected, and appears consistent for various mol ratios of benzenephosphonic acid to melamine.
It is an object of this invention to provide additives that will reduce the defects of corrosivity and toxicity inherent in the currently used fire-retardants.